In a typical base station of the prior art, local oscillator signals are provided for each one of the transceivers in the base station. Likewise, in a remote radio head application, individual local oscillator signals are also provided individually for each one of the transceivers located in the remote radio head application. It is necessary to provide a multiple number of individual local oscillator signals, since each one of the transceivers may be operating on different channels. The multiple numbers of local oscillators may also be included to improve reliability through the removal of the single point of failure which a single local oscillator would provide.
One issue associated with the approach of utilizing a multiple number of individual local oscillators is the expense and real estate on a chip associated with providing a plurality of local oscillators and the possible need to calibrate the different ones of the oscillators. This can be an issue during a start-up phase. For example, if the individual local oscillators are not correctly calibrated at the start-up, this may lead to difficulties in ensuring that the required beam forming operations for the radio signals are undertaken correctly. In particular, this may mean that the correct beam shapes for the radio signal in the required directions are not correctly calculated.
FIG. 1 shows an example of an active array system 1 as known in the prior art and comprising a plurality of transmission paths. Only a first signal path 16a at the top, a second signal path 16b in the middle and a last or n'th signal path 16n at the bottom are illustrated in FIG. 1 (as well as in the subsequent FIGUREs). The third to the (n−1)th transmission path are not illustrated for the sake of clarity.
A radio signal 10 in the digital domain to be transmitted reaches the active antenna array 1 from the left and is fed to the digital signal processor 15. The digital signal processor 15 distributes the radio signals to be transmitted to the plurality of output paths 16a, 16b, . . . , 16n. In the prior art example illustrated the radio signals 10 to be transmitted by the plurality of output paths 16a, 16b, . . . , 16n are digital IF transmission signals which have undergone upconversion in the digital signal processor 15. Other processes may also take place in digital signal processor 15 and these include, but are not limited to, crest factor reduction, digital predistortion and digital beamforming The inclusion or omission of these processes has no impact on the teachings of the disclosure as described herein. For simplicity the letters relating to all of the paths will be left out in future reference numerals.
Only the passage of the transmission signal through the top one of the output paths 16a will be described in detail. It will be appreciated that all of the other output paths 16b, . . . , 16n are identical. The output path 16 is connected to a digital-analogue converter 20 which converts the digital IF transmission signals from the digital signal processor 15 to analogue signals prior to passing the analogue signals through a first filter 25 to obtain those filtered transmission signals in the desired frequency band. The filtered transmission signals in the desired frequency band are forwarded to a first mixer 30. The first mixer 30 upconverts the filtered transmission signals by means of a first local oscillator 35 to an analogue intermediate frequency band.
The output of the first mixer 30 is filtered in a second filter 40 and passed to an intermediate frequency amplifier 45 for amplification. The output of the intermediate frequency amplifier 45 is passed to a second mixer 50 at which it is upconverted with an oscillator signal from the second local oscillator 55.
The transmission signals from the first mixer 50 are now at a transmission frequency band (i. e. the radio frequency) and are passed through a third filter 60 into a radio frequency amplifier 65 before entering a transmission filter 70 and being passed to the radio frequency output 80. The radio frequency output 80 is connected to one of the plurality of antenna elements from the antenna array (not shown). A tap 75 provides a feedback loop 76 to the digital signal processor 15 through paths 85 which allow calibration and updating of the predistortion processing of the radio signals to be taken into account.